Microscopic strain mapping using scanning electron microscopy topography image correlation at large strain

Measuring the distribution of local strain at the microscopic level is a challenging problem, especially for materials subjected to large overall strain. In the present study, a novel microscopic strain mapping technique has been developed based on the analysis of surface topography using digital image correlation (DIC) software. The input is a series of scanning electron microscopy (SEM) images. The method uses topographic features (such as surface slip traces) found in these images as the input. A commercially available optical strain measurement system (ARAMIS (R), which is a trade name of the equipment from GOM mbH, Braunschweig, Germany) that utilizes the DIC methodology is used for this purpose. It was found that the best results were obtained using an incremental approach in which DIC is used to map the local strain increments following a modest amount of macroscopic deformation. This is essential when using topographic features such as slip traces that are not static. The accuracy and scale of the measurements are affected by image and facet size. The method has been validated, based on in situ deformation of an aluminium alloy within an SEM, using strains measured independently by means of surface indents. The results clearly reveal the details of the local shear on a sub-grain-size scale and the evolution of shear bands within the necking area, leading to local strains that exceed the average strain by a factor of 2.3.